Evidence of amorphization in molecular-dynamics simulations on irradiated intermetallic NiAl

Molecular-dynamics (MD) computer simulations on three intermetallic compounds, NiAl, ${\mathrm{Ni}}_{3}$Al, and ${\mathrm{Cu}}_{3}$Au have been performed to investigate the kinetics of the disordering and amorphization processes. These systems were chosen because the embedded atom-type potentials work well for these materials and also because they have experimentally different amorphization behavior. Previous simulations of collision cascades with 5-keV Ni and Cu primary knockout atoms (PKA) have shown a complete loss of the crystalline structure but only partial chemical disorder in the core of the cascade. Dynamical melting simulations of the liquid phase provided significant differences in the short-range order between the three intermetallics, namely: (i) ${\mathrm{Cu}}_{3}$Au is close to an ideal mixture, ${\mathrm{Ni}}_{3}$Al is the most ordered liquid, and the disordering level of NiAl lies between the two ${\mathit{A}}_{3}$B intermetallics, and (ii) NiAl has the fastest and ${\mathrm{Cu}}_{3}$Au has the slowest kinetics in the disordering process after a sudden increase of temperature. For details see Spacz\'er et al. [Phys. Rev. B 50, 13 204 (1994)]. In the present paper we look for the conditions to induce amorphization in MD cascades in NiAl by 5 and 15 keV PKA's. The kinetic energy of the atoms in the simulated systems is removed on different time scales as a way to mimic strong or weak coupling between electrons and phonons. No evidence of amorphization is found at the end of the cascades created by 5 keV recoils. However, the 15 keV PKA events show that (i) in the no-coupling case the system evolves to a highly disordered state, (ii) an amorphous region with about 100 nonlattice atoms is found in the case of weak coupling, (iii) the locally molten and recrystallized region collapses to a small cluster containing 25 atoms when medium coupling is used, and (iv) a highly ordered state results in the case of strong coupling. Amorphization in MD cascades is reported. A 15 keV recoil event with weak electron-phonon coupling is also shown for ${\mathrm{Ni}}_{3}$Al. The final structure of the ${\mathrm{Ni}}_{3}$Al system shows no amorphous cluster formation in agreement with experimental results.